The present invention relates to a process for electrophotographic matrix printing, wherein a light beam is projected onto an array of electroptical opticalmodulators disposed along line segments and wherein this beam is recombined, after passage through said modulators, in one continuous line on a photosensitive support in motion.
The invention also relates to a device for carrying out this process, comprising a virtual pinpoint light source, an array of electrooptical modulators disposed along line segments and an optical guide for conducting light from the source to the modulator array in such a manner that the light traversing these modulators is recombined in a continuous line on a photosensitive drum in rotation.
Electrophotographic printers are based on selectively illuminating the photosensitive surface of a rotary drum by optoelectronic means. Writing is generally effected line by line in a direction substantially parallel to the drum axis. The succession of these lines, when the drum is in rotation, forms a matrix image of the graphic document to be reproduced.
The best known operating principle calls for a laser beam intensity modulated by an accousto-optical device and reflected by a rotating polygonal mirror so as to rapidly scan the writing line. This system, which has the advantage of great flexibility of speed and resolution, presents various drawbacks, among which are bulkiness linked to the great focal distances of the lens means for concentratating the beam on the scanned line, the cost of said lens means which must compensate focal length variations between the center and the extremities of the line, as well as the cost of making a high-precision mirror rotating at high speed.
It has been proposed to reduce these difficulties by replacing the scanning laser beam by a line of discrete elecrooptical elements, modulated in parallel. These elements may be active sources such as photoemitting diodes or passive components such as light modulators.
Such systems are intrinsically much more compact than laser printers and have the adavantage of comprising no mobile part. However, to obtain a sufficient resolution, of the order of 10 to 15 points per mm, the number of elements necessary for realizing a line is very high, in the order of 2000 to 3000 for a standard line of 210 mm. These elements must each be addressed to an electronic control circuit. Such a density of elements and circuits necessarily implies, for economical production, resorting to the techniques for fabricating integrated circuits. Now, it is known that the latter are cut out in the form of chips from wafer of a limited size which is currently situated at about 100 mm. In addition, the individual size of a chip is much smaller than that of a wafer, in order that the surface may be well filled on one hand and that the presence of manufacturing flaws which are statistically inevitable in a number proportional to the surface, may not cause the yield in utilizable chips to fall below a reasonable rate.
With regard to the electrooptical modulating elements, the maximum size of a monolithic line segment can hardly exceed 30 to 40 mm. Thus, to realize a complete line of 210 mm, one must have six to eight segments juxtaposed end to end. This presents the problem of continuity of the writing line. Each segment in effect physically presents an inactive border, consisting of the sawing and encapsulating edge, which interrupts the line by a dead space on juxtaposition.
Another problem, in the case of passive modulator lines, is that of conveying the light to be modulated up to the electrooptical elements.
These two problems have not been solved in a satisfactory manner. For example, a printing head with magnetooptical modulators is known, which is described in the article "Advances in Laser and Electroptic Printing Technology" by R. A. Sprague, J. C. Urbach and T. S. Fisli, published in the journal "Laser Focus" of October 1983, on page 101 and illustrated more particularly in FIG. 11, wherein light is conducted from a concentrated source to a row of modulator segments by means of a fiber-optical converter with fibers disposed in fan-shape. The aligned but discrete images of the line segments are then focussed slightly enlarged on the drum, so as to reform a continuous line by means of a series of large-diameter objectives, each corresponding to a segment. Both the fan-shaped sheaf of fibres consisting of several thousands of fibers to be aligned facing the modulators as well as the series of objectives constitute costly auxiliary equipment. The row of objectives is moreover bulky and adjustment of the images of the line segments is quite delicate.